Conventional rotation shafts provided in various drive force transmission mechanisms are rotatably supported via bearings. For such rotation shafts, so-called ball bearings are used in order to improve rotation performance of the rotation shafts. The ball bearings contain multiple rolling elements (rolling members) circumferentially arranged between inner and outer rings.
Such bearings are used as support members for rotation shafts of drive force transmission mechanisms in various drive units, and some drive units are desired to prevent foreign substances such as dust and water from entering inside through the bearings. Moreover, when foreign substances enter the bearings, rotation performance of the rotation shafts could be deteriorated and noise could be caused. For this reason, Patent Literature 1 for example discloses a magnetic fluid sealed bearing having a sealing ability using a magnetic fluid.
In the magnetic fluid sealed bearing disclosed in Patent Literature 1, a ring-shaped polar plate having a ring-shaped (annular) magnet attached thereto is mounted on (fitted in) one of an inner ring and an outer ring, a gap is formed on the other of the inner ring and the outer ring, and a magnetic fluid is retained in the gap portion, so as to prevent foreign substances from entering inside. In the magnetic fluid sealed bearing disclosed in Patent Literature 1, the ring-shaped magnet is magnetized in the axial direction, and the ring-shaped polar plate is attached to the opening side of the ring-shaped magnet. In this arrangement, a magnetic circuit is formed on each of the inner ring side and the outer ring side. Therefore, the magnetic fluid is also retained in a minute gap formed on the side where the ring-shaped polar plate (the ring-shaped magnet) is mounted (a gap formed on the mounting side will be hereinafter referred to as “the minute gap”), so as to prevent the foreign substances from entering inside from the outer ring side and the inner ring side.